Various arrangements have been suggested over the years to manage the energy consumption in buildings having a plurality of zones. These energy management systems can be very useful in many situations, for instance for peak load management and/or for energy conservation purposes. Examples of these arrangements are presented in U.S. Pat. No. 4,132,355 of 2 Jan. 1979 (Clear et al.) and in U.S. Pat. No. 5,625,236 of 29 Apr. 1997 (Lefebvre et al.), to name just a few.
An energy management system generally includes a remote console that can control a plurality of power-consuming apparatuses located in the various zones. An example of such apparatuses is a heating unit. Different approaches exist for controlling them. One of these approaches involves the use of switching devices, such as relays, that are each mounted in series on the local electric circuit feeding power to the apparatus. Each switching device is actuated from the remote console. The operation of the apparatus can be prevented or authorized by selectively opening and closing the electric circuit using the switching device.
In some implementations in which the apparatuses are heating units, an electronic thermostat is located in each zone to control the temperature. It includes an internal electronic circuitry providing functions that are not available with a bimetallic thermostat or a thermostat with a mercury switch. For instance, an electronic thermostat can be designed to automatically change the temperature set point at specific times of the day. It can also be designed to generate a “pulsed” heating so as to lower the energy output of the heating unit and thereby minimizing the temperature fluctuations in the zone. Pulsed heating can be achieved by switching the voltage on and off over a given period of time. This period of time is generally a few seconds, for instance 15 seconds. Nevertheless, using other time lengths is also possible. The proportion of “on” time over the given time period is called the duty cycle. A duty cycle of 100% means that the heating unit receives the full electrical power. However, a duty cycle of 25% corresponds to a situation where the power is “on” only 25% of the time, thus to a situation where the heating unit receives only 25% of the normal electric power during the given time period and generates only about 25% of the normal heat.
Various models of electronic thermostats exist for achieving pulsed heating. Some of them are designed to be mounted in series on one of the wires of an electrically powered circuit without the need for a neutral or ground wire. These electronic thermostats are able to draw the power needed to energize their internal circuitry directly from the single wire. However, this works as long as the electric circuit is closed, namely when the electronic thermostat is the only device controlling the electrical power fed to the heating unit.
When using a switching device mounted in series on a circuit as part of an energy management system, the circuit will be “opened” when the switching device is in its “open” position. This opened position will prevent the corresponding heating unit from operating, thereby completely overriding the normal control provided by the local electronic thermostat. However, sometimes even after only a few seconds, the internal circuitry of the corresponding electronic thermostats will run out of power since it needs the circuit to be closed to get its own power. When this happens, the electronic thermostat may no longer function as usual. The display of the electronic thermostat will disappear and, depending on the models, the user's settings of the thermostat can even be lost.
Switching devices can be used to open the circuits for long periods of time, even sometimes days or weeks, but some energy management systems can also use the switching devices to create a “pulsed” heating, even if the local electronic thermostats may also be designed to create a pulsed heating. This, however, may sometimes prevent the electronic thermostats from receiving the minimal power they need for energizing their internal circuitry, especially if each cycle of the pulsed heating controlled through the switching device is relatively long. For instance, in a case where the period is 10 seconds and the duty cycle is 50%, the circuit will be opened for 5 seconds during each period of 10 seconds. The internal circuitry of an electronic thermostat will run out of power during these 5 seconds unless it includes a capacitor or the like that is capable of keeping the internal circuitry energized for at least the 5 seconds the circuit is open. If not, it will run out of power once during every cycle, thereby causing its display screen to fade, blink or disappear at each cycle. Other problems may also be experienced with the electronic thermostats in this case.
Some electronic thermostats are provided with a backup battery or even with other auxiliary backup power arrangements, for instance a dedicated power line. However, many models are not and this can become a challenge during the installation of an energy management system in an existing building. In fact, changing existing electronic thermostats that are otherwise working perfectly well is not necessarily desirable for many reasons, costs being one of them. Still, using a backup battery may not solve the problem, especially when a switching device opens the circuit for a long time or when battery power would be needed at frequent time intervals since the battery will eventually run out of power as well.
Clearly, room for improvements still exists in this area.